meaning - Dust vs. Undust?

The entry for "dust" from LDOCE says:

dust¹ (n.)

1. 
   

   [uncountable] → HOUSEHOLD dry powder consisting of extremely small bits of dirt that is in buildings on furniture, floors, etc. if they are not kept clean:

   
   

   All the furniture was covered in dust.

   
   


2. 
   

   [uncountable] → HOUSEHOLD ...

   
   


3. 
   

   [uncountable] → INDUSTRY, HARD SCIENCE ...

   
   


4. 
   

   a dust (BrE) → CLEANING the act of dusting something:

   
   

   I need to give the sitting room a dust.

   
   


5. 
   

   ...

   
   

---

dust² (v.)

1. 
   

   → CLEANING to clean the dust from a surface by moving something such as a soft cloth across it:

   
   

   Rachel dusted the books and the bookshelves.

   
   


2. ...


3. ...

Now, the entry for "undust" from Wiktionary says:

undust (v.)

1. (obsolete) To free from dust.

I do notice the entry being marked as "obsolete" for undust, and also I remember my English classes as a child where we had a rectangular cubic sponge called a "duster" that we used to dust/undust the chalk on the blackboard, however every time I'm cleaning my desk, computer, furniture, etc. I'll automatically think of "undusting" and then will force myself to undo and say "dusting" instead, just to convince myself that the latter is the grammatical one; however I can't see the logic behind this verb and its usage based on general English grammar.

Google Ngrams also shows up some results (even if not many) for undust:

Is undust incorrect? Did it exist as a correct verb years ago, and can I still use it?

I feel paranoid to say: "I'm dusting my keyboard!" because it makes me feel that it has the opposite meaning, as if I'm spreading dust and particles on my keyboard, especially since these chores aren't all that commonly spoken with today's lifestyle, and stating the former sentence might sound a bit odd.

P.S. Note that "dust something off" is slightly different in usage e.g. "They were dusting off leaves and twigs.", and based on my understanding from the definitions in dictionaries, saying "I'm dusting my keyboard off." isn't correct.

Answer

Undust is used so little that you should indeed see it as obsolete. That ngram shows some results is not really relevant if you compare it with the occurences of dust.

Your paranoia is uncalled for, there is really no proficient, let alone native, speaker of English that will think that dusting your keyboard is similar to watering your plants.

I can understand where your hesitation to use it comes from, as you seem to be saying to opposite of what you are doing. Actually, to complicate matters, dust is used in the sense of adding dust as well: lightly dust the cake form with flour.

In context, however, there will usually be absolutely no confusion. When you dust your keyboard, everyone will understand that you are cleaning it.

As to the logic behind one word meaning two different (opposite) things, we are talking about English, the language that uses words like inflammable, and in which people say I could care less when they mean the opposite. Don't get stuck too much on logic when it comes to natural language!